(i) Field of the Invention
This invention relates to a method for producing an InSb thin film element whose characteristics are improved by heating above the melting point of InSb. More particularly, it relates to a method for producing an InSb thin film element in which an InSb thin film has a thickness of at most 0.2 .mu.m and exhibits good electrical characteristics.
(II) Brief Description of the Prior Art
In resent years, as the computing speed of an electronic computer has become higher, it has been required to render higher the operating sped of transistors for use in the electronic computer. Therefore, also a MOS-IC for the electronic computer need have the operating speed made higher. Since, in general, the operating speed of a transistor is substantially proportional to the electron mobility of a constituent semiconductor material thereof, a MOS type FET constituted of a semiconductive material of high electron mobility comes to be desired. The electron mobility of silicon (Si) having heretofore been employed for transistors is several hundreds cm.sup.2 /V.sec. It must be said that, insofar as Si is used, it is subject to the limitation to render the operation of a transistor highly speedy. On the other hand, our experiments have revealed that an InSb film having a thickness of, for example, about 1.5 .mu.m exhibits as high an electron mobility as 6 .times. 10.sup.4 cm.sup.2 /V.sec. A MOS type FET employing the InSb film is expected to realize a speed exceeding the limitation of the operating speed of the conventional Si-MOS type transistors.
With the MOS type FET employing the InSb film, however, when the thickness of the InSb film is greater than about 0.2 .mu.m, an electric field normal to the film surface is not effectively applied by the provision of a gate electrode, and the operation as an FET is difficult. Therefore, an InSb thin film whose thickness is about 0.2 .mu.m or below and which has good electrical characteristics, especially an excellent electron mobility, has come to be desired. However, it has hitherto been extremely difficult to acquire such InSb thin film.
It has heretofore been well known that an InSb film whose thickness is in the order of 1 .mu.m is deposited on a substrate and thereafter once molten to execute (1) the larger regrowth of crystallite and (2) the purification, whereby the electron mobility of the InSb film can be enhanced. It has also been known that, if there is not any covering film on the InSb film in that case, at above the melting point (525.degree. C.) of the InSb crystal (1) the melt of InSb will agglomerate to rupture the film and (2) Sb of the components of InSb as has a higher vapor pressure is selectively vaporized to deviate from stoichiometry. As the covering film for eliminating the drawbacks, an oxide film containing In.sub.2 O.sub.3 has been employed (A. R. Billings; J. Vac. Sci. Tech., vol. 6 (1969), page 757). Since, however, the oxide film is soft, corrugations arise in the InSb film when InSb recrystallizes from the melt. Unless the corrugations attributed to the crystal growth of InSb are suppressed, the thickness of the InSb film on the substrate does not become uniform. When, to the end of preventing this inconvenience, a hard vitreous covering film is deposited on the InSb film and the InSb film is molten, the glass film cracks. When the thin film whose thickness is at most 0.2 .mu.m is formed with the corrugations as stated above and its surfaces becomes uneven, the production of the element becomes difficult. However, unless the recrystallization being the cause for the development of the corrugations is carried out, the electron mobility of the InSb film is very low and such InSb film can not be put into practical use.